This invention is directed to an automatic slack adjusting mechanism for a cam actuated brake assembly, and, more specifically, to a lever actuator and mechanism which automatically adjusts for brake shoe wear in response to movement of the lever and related linkage beyond a predetermined acceptable distance.
Various types of cam actuating levers including automatic slack adjusting mechanisms for cam actuated brake systems are known in the prior art. One such automatic adjuster which employs a torque limiting one way drive mechanism to angularly adjust a camshaft relative to an actuating lever during brake application movement of the lever is disclosed in U.S. Pat. No. 3,507,369 which issued to J. L. Oliver on Apr. 21, 1970. Another automatic adjuster employing a one way drive operative upon brake application movement of the lever is disclosed in U.S. Pat. No. 3,526,303 which issued to C. L. Lodjic et al on Sept. 1, 1970.
An automatic slack adjuster which employs a one way driving connection to effect automatic adjustment upon return or brake releasing movement of the lever is disclosed in U.S. Pat. No. Re: 26,965 which issued to Sander et al on Oct. 13, 1970. Although adjustment is effected during the brake releasing stroke, this automatic adjuster employs an external cam in the main embodiment which is open to road dirt and requires space for movement in a plane normal to the direction of movement of the actuating rod. Since the actuating rod drives the cam, the position of the adjuster relative to the actuating rod must be properly fixed at the time of installation.
Other actuating lever and automatic slack adjusting mechanisms which effect adjustment after a portion of the mechanism contacts an abutment fixed to the vehicle axle or frame are disclosed in U.S. Pats. Nos. 3,121,478 to J. L. Bostwick, issued Feb. 18, 1964; 3,351,164 to S. Svensson, issued Nov. 7, 1967 and 3,371,755 to J. W. Leeper, issued Mar. 5, 1968.
Most of the previously known automatic slack adjusting mechanism for cam actuated brake mechanisms have been subject to certain limitations. Some previously known automatic adjusting mechanisms have been unable to distinguish between the brake shoe movement required to take up the desired running slack and further movement of the cam which results from distortion of the brake assembly. An adjustment based on the total arcuate movement of the lever or cam without providing for distortion in the system could result in over adjustment and undesirable rapid wear of the lining secured to the brake shoes.
Prior art adjusting mechanisms which must be mounted in some fixed position relative to the actuator motor or a separate member such as an abutment fixed to the vehicle axle require an additional amount of skill and inspection to insure that the adjuster is properly installed. Such adjusters also occasionally require additional design considerations for proper installation on particular vehicles.
The use of other prior art slack adjusters on certain vehicles has been hampered by their size and the amount of space required to effect movement of the lever and automatic adjusting mechanism within the confines of the axle and wheel assembly environment.
Still other prior art automatic adjusters have been hampered through the use of external operating components such as cams or ratchets which are exposed to road dirt and hazards and may be adversely effected thereby.
Other automatic slack adjusters which are operative to effect adjustment during brake applying movement of the lever must accommodate stresses resulting from the combined effect of the brake actuating lever movement and the incremental movement effected by the automatic adjusting mechanism. Mechanisms of this type may fail to make an adjustment when it is desirable. The lever movement driving through the adjusting mechanism may also shorten the effective life of the automatic adjusting mechanism.